1) Field of the Invention
The present invention relates to a voltage step down type DC-DC converter having a coupled inductor, which is preferably used in case it is required that a voltage should be largely lowered, for instance, used for a power supply apparatus where a voltage of 1.6 V is obtained from a power source with a voltage of 20 V.
Such a voltage step down-type DC-DC buck converter is generally used for the MPU (Microprocessor Unit) in the recently developed personal computers. The MPU is driven with a low voltage, for instance 1.6 V, however it requires a large current. Particularly, for lap-top type personal computers, it is required to make the efficiency higher while keeping the above condition in order to make the operating time of the battery long and to reduce heat generation.
2) Related Art
In the field of the power supply apparatus for personal computers where it is required to largely drop the voltage down, normal type buck converters have mainly been used. FIG. 1 is a circuit diagram showing a basic construction of the conventional buck converter. However, according to the conventional buck converters, when the voltage is greatly lowered, the duty ratio of a main switch becomes very small. Therefore, it becomes difficult to control the converter and the switching loss of the main switch becomes great. In order to solve this problem, it has begun common to use a voltage step down type converter where the voltage is decreased in two steps; an intermediate voltage is generated in a first step and a finally demanded low voltage is obtained in a second step. However, such a type of converter has problems in that the circuit thereof becomes complicated and also it is difficult to make the apparatus compact.
The present invention has for its purpose to provide a DC-DC converter, where the duty ratio of a main switch is made broader to make it easy to be controlled and a high efficiency can be realized by decreasing the switching loss of the main switch. According to the DC-DC converter of the invention, it is possible to largely lower the voltage, while the complexity caused in the conventional buck converter where the voltage is decreased in two steps is withdrawn, the size of the apparatus becomes compact, and the cost for manufacturing can be saved.
In order to realize the purpose, the DC-DC converter according to the present invention has a characteristic in that the DC-DC converter comprises a DC power supply, a main switch, a free-wheel inductor and a free-wheel switch, wherein an intermediate tap is provided in the free-wheel inductor, thereby a free-wheel current goes out through the intermediate tap. According to this construction, the duty ratio of the main switch becomes broader in comparison to the conventional buck converter that does not have a tap in the free-wheel inductor.
The DC-DC converter according to the present invention has an aspect in that an N-channel FET is used for the free-wheel switch, while a P-channel FET or a PNP transistor is used for the main switch. According to this aspect, the driving circuit for both the switches can be made simple.
The DC-DC converter according to the present invention has another aspect in that an N-channel FET or an NPN transistor is used for the main switch being combined with a pulse transformer. According to this aspect, the driving circuit for the main switch can also be made simple.
The DC-DC converter according to the present invention has still another aspect in that the DC-DC converter further comprises one or more loss-less snubber circuits, where a surge energy generated from a leakage inductance, due to the existence of the intermediate tap, is reused, so that an energy recovery is realized. The DC-DC converter may further comprise a sub-switch for conducting a soft switching operation. According to the present invention, the driving circuits for driving the main switch and the free-wheel switch, the active snubber circuit and the sub-switch for the soft-switching operation can be made simple.
The DC-DC converter according to the present invention has still another aspect in that in case a sufficiently high input voltage can be obtained, both the driving circuit for the main switch (high side) and the driving circuit for the free-wheel switch (low side) may be connected together in series, so that the power for driving can be saved.
Furthermore, the DC-DC converter according to the present invention has still another aspect in that the DC-DC converter has a construction where a sub-switch for ZVT (zero voltage transition) operation is added to the construction of a flyback converter so that each switching element there conducts a soft switching operation. Furthermore, one or more loss-less snubber circuits is added to the construction to realize a more preferable operation.
The DC-DC converter system according to the present invention has still another aspect in that two or more DC-DC converters having the characteristics mentioned above are connected together in parallel so that so-called interleave construction is realized, driving signals are driven in a multi-phase manner. According to this interleave construction, it is possible to reduce noise and/or ripples in the output of the converter.